# How fast does the overhead fan rotate?

sonhouse
Posers and Puzzles 14 Feb '11 17:33
1. sonhouse
Fast and Curious
14 Feb '11 17:33
So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the blade, which is normally 150 mm wide, is blurry, looks twice as wide. So with that info, how fast is the blade turning?
2. 14 Feb '11 19:17
1.5 hamster per second?
3. forkedknight
Defend the Universe
14 Feb '11 21:473 edits
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

If the fan blade moves 150mm in 100ms, then the linear velocity of that section of fan blade is 1.5m/s. It is not possible to calculate the rotational velocity of the fan with the given information.
4. 14 Feb '11 21:58
Originally posted by forkedknight
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
i told you, it is the hamster!
5. sonhouse
Fast and Curious
15 Feb '11 01:191 edit
Originally posted by forkedknight
I don't know about you, but I certainly can't blink my eyes 10 times/second. Therefore I think the question as posed is accurate.

If you *could* blink your eyes 10x/second, and assuming the fan has 5 blades, it would be spinning approximately 2 RPM.

If the ...[text shortened]... It is not possible to calculate the rotational velocity of the fan with the given information.
I got that after I posted. The diameter of the fan blades is 1.3 meters. You only need one blade to make the calculation.
6. 15 Feb '11 08:402 edits
Originally posted by sonhouse
So I know if I blink my eyes as fast as I can, I am making 100 millisecond clips of time. So I blink my eyes like that at my overhead fan, and I notice I can stop the blades ala strobe effect. However, the blades are not crisp images like those strobes which flash for a few microseconds, my eyes are open for 100 millisecond clips. So in that 100 ms, the bla ...[text shortened]... is blurry, looks twice as wide. So with that info, how fast is the blade turning?so it is moving
In 100 milliseconds the blade tip moves 150 mm

so, converting to metres and seconds:

Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s

blade circumference = 1.3 * pi metres

so tip will travel circumference in

1.3 * pi / 1.5 seconds

and the rotational speed is 1.5 /( 1.3 * pi) s^-1

= 0.367 rotations per second, a slowly rotating fan.

The extra constraint is that the blades must appear stopped.

You are could be closing your eyes for 2.6 seconds, which is what would be needed for the first blade to come round again (( 1.3 * pi - 0.15)/1.5 = 2.6), but that sounds a bit long...

So perhaps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
7. sonhouse
Fast and Curious
15 Feb '11 09:521 edit
Originally posted by iamatiger
In 100 milliseconds the blade tip moves 150 mm

so, converting to metres and seconds:

Velocity of tip = 0.15 / 0.1 m/s = 1.5 m/s

blade circumference = 1.3 * pi metres

so tip will travel circumference in

1.3 * pi / 1.5 seconds

and the rotational speed is 1.5 /( 1.3 * pi) s^-1

= 0.367 rotations per second, ...[text shortened]... aps it is a 4 bladed fan and you are closing your eyes for about 0.66 seconds between snapshots?
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
8. 15 Feb '11 10:19
Originally posted by sonhouse
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one ...[text shortened]... perceived blurring width could be used to calculate the RPM's within certain limits of course.
you seem to spend a lot of time looking at that fan.

you might enjoy this:

now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?

π
9. sonhouse
Fast and Curious
15 Feb '11 11:58
Originally posted by tharkesh
you seem to spend a lot of time looking at that fan.

you might enjoy this:

now, giving the calculated speed and a cord length of 0.5m, and the weight of the cat of around 5kg, how much force has the wall be able to withstand?

π
OMG that cat was is serious trouble. My guess is it wouldn't do that againπ
I guess I am a fan of fan'sπ
10. smw6869
Granny
15 Feb '11 19:12
Originally posted by tharkesh
i told you, it is the hamster!
Yes, but doesn't it depend on whether the hamster's buttocks muscle is slow twitch or fast twitch?

GRANNY.
11. forkedknight
Defend the Universe
15 Feb '11 21:431 edit
Originally posted by sonhouse
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
If the blade "appears" to be 300mm, it just means that the front edge is being replaced by the back edge, so the distance traveled is only a single width of the blade, or 150mm.

I concur with iamatiger's estimate of .367 rps which translates to ~22 RPM
12. 16 Feb '11 00:202 edits
Originally posted by sonhouse
Yes, it is a four bladed fan. Using the tip size of 1.3 meters, I got about 45 RPM.
I reasoned the blade as seen is blurred to twice its real size, or 300 mm, so covering twice the distance in 100 ms. In order to get the exact speed with a strobe, you would have to have two microsecond strobe pulses spaced 100 ms apart to give two blade widths because one perceived blurring width could be used to calculate the RPM's within certain limits of course.
If you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.
13. sonhouse
Fast and Curious
18 Feb '11 18:15
Originally posted by iamatiger
If you did that with the strobe, it wouldn't appear stopped, the blades would appear to be advancing one blade width every flash, unless the blades were only separated by a blades width, but that would be a lot of blades.
That would depend on the pulse rate. For instance, you could set up the strobe to do two quick pulses just the right time followed by a longer period, two pulses in a row with a time gap then two more pulses, you could show the same blade one blade width apart. Having said that, that is not the standard way a strobe works, it only has one pulse per period. I suggest two pulses per period would do the trick.
14. smw6869
Granny
18 Feb '11 18:54
Aren't the RPM of the fan stamped in to the hamster's left ear at the factory? Just saying.

GRANNY.